Known in the art is an internal combustion engine arranging a denitration catalyst in an engine exhaust passage, feeding air containing ammonia upstream of the denitration catalyst to make ammonia be adsorbed at the denitration catalyst, and using the adsorbed ammonia to reduce the NOX in the exhaust gas (for example, see Japanese Patent Publication (A) No. 2006-257936). In this internal combustion engine, to make the NOX reduction action by the denitration catalyst start right after engine startup, when the engine should be started, before starting the engine, the denitration catalyst is preheated and, after the denitration catalyst is activated, the engine is started.
In this regard, in the past, an NOX selective reduction catalyst comprised of for example Fe zeolite was used for reducing NOX in the presence of ammonia. In this case, usually, an aqueous urea solution is fed into the engine exhaust passage upstream of the NOX selective reduction catalyst, the ammonia produced from the aqueous urea solution is made to be adsorbed at the NOX selective reduction catalyst, and the ammonia adsorbed at the NOX selective reduction catalyst is used to reduce the NOX in the exhaust gas.
Now then, usually, in an internal combustion engine, when there is a request to start the engine, it is necessary to immediately start the engine. Further, an NOX selective reduction catalyst rises in temperature far more easily than the above denitration device. Therefore, when a NOX selective reduction catalyst is used, usually, when there is a request to start the engine, the engine is immediately started and the temperature of the NOX selective reduction catalyst is caused to rise by the exhaust gas exhausted from the engine.
In this way, if the temperature of the NOX selective reduction catalyst is raised and the NOX selective reduction catalyst is activated, the NOX reduction action is started. At this time, to reduce the NOX well, a sufficient amount of ammonia has to be adsorbed over the entire NOX selective reduction catalyst. For this reason, it may be considered to feed aqueous urea solution to the NOX selective reduction catalyst right after engine startup. However, right after engine start, the temperature at the wall surfaces inside the exhaust passage is low, so the fed aqueous urea solution continues to stick on the wall surfaces inside the exhaust passage in a liquid form or, because the exhaust gas temperature is low, the aqueous urea solution is not sufficiently atomized, so it is difficult to get all of the fed aqueous urea solution to be adsorbed in the form of ammonia at the NOX selective reduction catalyst as a whole. That is, at the time of engine start, even if feeding aqueous urea solution, it is not possible to reduce the NOX well early on.